Magnetically actuated torsional micro-mechanical mirror system

ABSTRACT

A torsional micro-mechanical mirror system includes a mirror assembly rotatably supported by a torsional mirror support assembly for rotational movement over and within a cavity in a base. The cavity is sized sufficiently to allow unimpeded rotation of the mirror assembly. The mirror assembly includes a support structure for supporting a reflective layer. The support structure is coplanar with and formed from the same wafer as the base. The torsional mirror support assembly includes at least one torsion spring formed of an electroplated metal. An actuator assembly is operative to apply a driving force to torsionally drive the torsional mirror support assembly, whereby torsional motion of the torsional mirror support assembly causes rotational motion of the mirror assembly. In another embodiment, a magnetic actuator assembly is provided to drive the mirror assembly. Other actuator assemblies are operative to push on the mirror assembly or provide electrodes spaced across the gap between the mirror assembly and the base. A process for fabricating the torsional micro-mirror is provided. The torsional micro-mirror is useful in various applications such as in biaxial scanner or video display systems.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit under 35 U.S.C. § 119(e) ofU.S. Provisional Application No. 60/057,700, filed on Aug. 27, 1997, andunder 35 U.S.C. § 120 of U.S. application Ser. No. 09/138,367 filed onAug. 26, 1998, the disclosures of which are incorporated by referenceherein.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

[0002] The invention was made with Government support under Contract No.DAAK60-96-C-3018 awarded by the Soldier Systems Command of the UnitedStates Army. The Government has certain rights in the invention.

BACKGROUND OF THE INVENTION

[0003] Micro-electromechanical system (MEMS) mirrors (or micro-mirrors)have been evolving for approximately two decades as part of the drivetoward integration of optical and electronic systems, for a range ofuses including miniature scanners, optical switches, and video displaysystems. These structures consist of movable mirrors fabricated bymicro-electronic processing techniques on wafer substrates (for examplesilicon, glass, or gallium arsenide). The torsional micro-mirrortypically comprises a mirror and spring assembly suspended over a cavityformed in or on a base. The mirrors are electrically conductive, as isat least one region behind the mirror, affixed to the stationary base,so that an electric field can be formed between the mirror and the base.This field is used to move the mirror with respect to the base. Analternative comprises the use of magnetic materials and magnetic fieldsto move the mirrors.

[0004] Typically the mirror surface consists of either the wafer itselfor a deposited layer (metal, semiconductor, or insulator), and generallyin the prior art the springs and mirror are formed from the samematerial (but not in all cases). The mirror and torsion springs areseparated from the base by an etch process, resulting in the formationof a cavity between the mirror and base.

[0005] For display or image acquisition applications, the goal is todevelop compact systems with rapid frame rates (at least 60 Hz) and highresolution, consisting of between 200 and 2000 miniature pixels perline. For scanning system designs in this range, the mirrors should belarge (in the range of 200 μm×200 μm to 2 mm×2 mm), fast (in the rangeof between 3 kHz and 60 kHz for resonant devices), and scan a photonbeam through a large angle 7 to 40 degrees).

[0006] In optical systems that contain very small elements, diffractionby the smallest element may introduce diffraction broadening anddeleteriously increase the final pixel size. Enlarging the limitingelement reduces this broadening and militates for larger mirrors.However, as mechanical systems get larger (for example, increasing thesize of a torsional mirror), they tend to be characterized by greatermass and consequently lower resonant frequency; this resonant frequencysets the scanning speed of the system. A frequency in the range of 5 to50 kHz is desirable. Prior art mirror designs have been limited by thedifficulty inherent in obtaining a high resonant frequency with a largemirror size, free from diffraction broadening effects. In prior artcases in which the mirror mass is made very low to obtain high resonantfrequency, the resultant reduction in stiffness of the mirror is alimiting factor in the quality of the reflected image. This problem isexacerbated by the possibility of heating of the mirror by lightabsorbed in the mirror. Such heating militates for a thick mirrorcapable of conducting the heat away from the source.

[0007] The scanning angle through which the mirror moves determines thenumber of distinguishable pixels in a display or imaging system.Therefore, a large scanning angle is desirable. Generally in the priorart the scan angle is limited by the presence of electrodes thatinterfere with mirror motion (but not in all cases).

[0008] Electrostatic actuation is the most common method used to drivemicro-mirrors. In order to produce a force, a voltage is generatedbetween two electrodes, usually the plates of a parallel platecapacitor, one of which is stationary and the other of which is attachedto the mirror as described previously. By making the mirror anelectrical conductor, the mirror itself can be made to serve as one ofthe plates. The force generated for a given voltage depends on the platearea and on the gap between the plates, which may change as the mirrorposition changes. For torsional mirrors, the important drive parameteris the torque, and the effective torque on the structure is alsoproportional to the distance between the resultant force and the axis ofrotation of the mirror. Thus, a large driving force can be achievedusing large capacitor plates and small gaps; by applying the force at adistance from the rotation axis, a large torque may be obtained.

[0009] In many prior art designs the criteria for a large deflectionangle range tend to be in conflict with the criteria for large drivingforces. The deflection angle is limited by the presence of surfacesbehind the mirror. An example of a limiting surface would be the bottomof a cavity in the base etched beneath the mirror, or some othersubstrate on which the mirror is mounted. The maximum angle is achievedwhen the mirror contacts this backplane, so the small separation betweenthe mirror and the backplane needed for generating adequateelectrostatic deflection force limits the maximum angle. Accordingly, inprior art designs in which the mirror is used as one of the driveelectrodes and the other electrode is on the backplane, increasing thegap reduces the force or torque obtained at a given voltage. Some priorart designs use electrodes that are offset from the main mirror body andwhich are connected through actuator linkages, allowing the backplane tobe moved further away or even eliminated entirely. Typically, though,these electrodes have smaller active areas and shorter moment arms,which tend to reduce the effective forces and torques as well.Additionally, if as the mirror moves, the gap between the driveelectrodes narrows, then the gap still may be a limiting factor for therange of motion of the structure.

[0010] A second set of design problems arises in the selection of themirror. Prior art designs and processes do not permit the mirror to bemade from very low mass material without also sacrificing structuralrigidity. One of the process limitations is the use of the same materialfor torsion spring and mirror mass, or the same set of patterning stepsfor spring and mirror mass. The selection of mirror materials with aview toward the elastic or fatigue properties of the springs restrictsthe suitability of the material with respect to mirror mass rigidity,and also limits the optical performance of the mirrors.

[0011] In 1980, Peterson disclosed a silicon torsional micromachinedmirror (U.S. Pat. No. 4,317,611; K. E. Peterson, “Silicon torsionalscanning mirror,” IBM J. Res. Dev., 24(5), 1980, pp. 631-637). Both themirror and torsion elements were patterned in a thin (134 microns)silicon wafer and retained the full thickness of the wafer. Thestructure was then bonded to a glass substrate, over a shallow well toallow room for the mirror motion. Actuation of the device waselectrostatic. The mirror body was used as one electrode and the otherelectrodes were placed at the bottom of the well under the mirror. Anarrow ridge in the well under the axis of rotation of the mirror wasused to eliminate transverse motion of the structure. The manufacturingprocess for this device was relatively simple, requiring a singlepatterning step for the silicon and two patterning steps for the glasssubstrate. Its resonance frequency was about 15 kHz, and at resonancethe angular displacement reached about 1°. The limitations of thisdevice are related to the depth of the well. A 2 mm mirror touches thebottom of a 12.5 μm well at a displacement of 0.7° (1.4° total motion).Increasing the well depth to increase the range of motion is notnecessarily desirable, because it proportionally reduces the torqueachieved for a given voltage.

[0012] Nelson (U.S. Pat. No. 5,233,456), Baker et al (U.S. Pat. No.5,567,334), Hornbeck (U.S. Pat. No. 5,552,924), and Tregilgas (U.S. Pat.Nos. 5,583,688 and 5,600,383) have developed and patented a series oftorsional mirror designs and improvements for use in deformable mirrordevice (DMD) displays. These mirrors are fabricated by surfacemicromachining, consisting of a series of patterned layers supported byan undisturbed substrate. The DMD display uses an individual mirror ateach pixel. The mirrors are therefore designed to be very small, to beoperated in a bi-stable mode, and to maximize the packing fraction onthe surface of the display. To minimize the gaps between the reflectingsurfaces of adjacent mirrors, the support structure and drive componentsare fabricated in underlying layers, requiring a complicated multi-stepdeposition and patterning process. As with the Peterson mirror, theHornbeck mirror is designed to serve as one of the deflectionelectrodes, and the others are placed behind the mirror. Owing to thesmall size of the mirrors (about 20 μm×20 μm), high deflection anglesare attainable with reasonably small gaps. These mirrors are designedfor driving at low frequencies, and for significant dwell at a givenangle (on or off), rather than for continuous motion, although the earlydevelopment included mirrors designed for resonant operation (U.S. Pat.No. 5,233,456). A scanned display or imager requires, however, a largemirror, and the difficulties with scaling up torsional mirrors that aredriven electrostatically with plates mounted behind the mirrors preventthe Hornbeck mirrors from being easily modified for use in scanningdisplay applications Toshiyoshi describes a silicon torsion mirror foruse as a fiber optic switch (H. Toshiyoshi and H. Fujita, “Electrostaticmicro torsion mirrors for an optical switch matrix,” J.Microelectromechanical Systems, 5(4), 1996, pp. 231-237). The Toshiyoshimirror is a relatively large device (400 μm on a side and 30 μm thick),which rotates about an axis close to one edge of the mirror. The mirroris defined by etching the silicon wafer from the front, and the excesswafer material is etched from the back of the wafer. It is thussuspended over a cavity in the wafer, supported by very thin (0.3 μm)metal torsion rods. The structure is then bonded onto another substrate,on which electrodes have been plated. Toshiyoshi has demonstratedseparation of the mechanical properties of the springs and mirror byusing silicon for the mirror mass, and metal for the springs. Actuationis electrostatic, by placing a voltage between the mirror body and theelectrodes of the lower substrate. The range of motion is limited by themirror hitting the glass substrate, at about 30°. In order to obtain themaximum deflection at an applied voltage of 80 volts, the stiffness ofthe torsion members must be very low, achieved by making them very thin.This also limits the resonant frequency of the structure to 75 Hz,making the approach unsuitable for a scanned display or scanned imager.Thus Toshiyoshi has not shown how the separation of the mechanicalproperties of the spring and mirror can be used to attain a highresonant frequency and high angular displacement.

[0013] Dhuler of the MCNC has disclosed a mirror wherein the mirror bodyis formed from the silicon substrate, while the supports and actuatorsare fabricated above the mirror plane using surface micromachinedpolycrystalline silicon layers (V. J. Dhuler, “A novel two axis actuatorfor high speed large angular rotation,” Conference Record of“Transducers '97,” 1997). The mirror body is first defined using ionimplantation of boron as an etch stop, and then by removal of the excessSi wafer from the back of the mirror. The supports and drive electrodesare offset from the top surface of the substrate by posts, which definethe gap between the drive capacitor plates. Thus the mirror is free torotate unhindered by the bottom surface of a well, while the drivetorque, being applied by actuators, is not limited by a requirement fora large capacitor gap. While it represents a significant advance in thestate of the art, this device suffers from certain flaws which thecurrent invention resolves.

[0014] In the MCNC process the mirror body thickness is limited by theboron implantation process, which has limited penetration depth; thedisclosed mirror was 4 μm thick. The stiffness of the mirror is limitedby both its size and thickness, so larger mirrors need to be thicker toavoid deformation of the mirror surface in use. For scanningapplications, flexure in the mirror leads to uncertainty in the pixelsize and location and distortion of the pixel shape. The implantationprocess also introduces stress into the mirror body, causing deformationof the reflective surface. The supports and actuators of the MCNC deviceare formed in a multi-step process and, as they are non-conducting,require the separate deposition and patterning of electrodes.

[0015] Kiang describes a 200 μm×250 μm mirror that has a frequency of 15kHz and maximum displacement of 150 (M. H. Kiang, “Surface micromachinedelectrostatic comb driven scanning micromirrors for barcodeapplications,” 9th Annual Workshop on Micro Electro-Mechanical Systems,1996, San Diego, Calif., pp. 192-197). This mirror is made of depositedand patterned surface layers, and before using it must be first rotatedout of the plane of the substrate using a comb drive and locked intoposition using complicated hinges. This approach obviates the problem offorming a cavity behind the mirror. However, the use of surfacemicromachined layers means that the structural rigidity of themicro-mirror cannot be controlled (because the thickness is limited tothin (a few microns) layers). The mirror motion is obtained byelectrostatic drive applied by an actuator linked to one edge of themirror. The motion of the mirror is restricted by the actuationmechanism.

[0016] Other torsional micromirrors are mentioned in the literature (M.Fischer, “Electrostatically deflectable polysilicon torsional mirrors.”Sensors and Actuators, 44(1), 1996, pp. 372-274; E. Mattsson, “Surfacemicromachined scanning mirrors,” 22d European Solid State DeviceResearch Conference, Sep. 14-17, 1992, vol. 19, pp. 199-204). Most aresmall (less than 100 μm on a side) and have very small displacements,not suitable for scanning applications. The exceptions tend to becomplicated to fabricate or actuate and suffer from the sameshortcomings as the mirrors described above.

[0017] Magnetically actuated cantilevered MEMS mirrors have beendisclosed by Miller et al. of the Califormia Institute of Technology (R.Miller, G. Burr, Y. C. Tai and D. Psaltis, “A Magnetically Actuated MEMSScanning Mirror,” Proceedings of the SPIE, Miniaturized Systems WithMicro-Optics and Micromachining, vol. 2687, pp. 47-52, January 1996; R.Miller and Y. C. Tai, “Micromachined electromagnetic scanning mirrors,”Optical Engineering, vol. 36, no. 5, May 1997). Judy and Muller of theUniversity of California at Berkeley disclosed magnetically actuatedcantilevered structures which may be used to support mirrors (Jack W.Judy and Richard S. Muller, “Magnetic microactuation of polysiliconflexure structures,” Journal of Microelectromechanical Systems, 4(4),December 1995, pp. 162-169). In both cases, the moving structures aresupported by cantilever beams along one edge. They are coated with amagnetic material, and upon the application of a magnetic field at anangle to the mirror surface, the mirror rotates in the direction of thefield, bending the cantilevers. Miller has also disclosed a similarmirror which uses a small coil fabricated on the moving structure toprovide it with magnetic moment. In Miller's mirror, the springs areformed out of the original silicon wafer, and in Judy's mirror thesprings are fabricated out of a polysilicon layer deposited for thepurpose. The conduction path for the magnetic coil device is provided bya separate NiFe contact.

SUMMARY OF THE INVENTION

[0018] The invention relates to micro-machinedoptical-electro-mechanical systems (MOEMS), and, more particularly, toresonant and non-resonant torsional micro-mirrors and their method offabrication.

[0019] The principal embodiment of the present invention comprises amirror assembly rotatably supported over a cavity in a substrate orbase. A torsional mirror support assembly is provided comprisingtorsional suspension springs and force pads attached to the springs andto the base. Actuation of the mirror is achieved by torsionally drivingthe springs via the force pads, thereby causing rotation of the mirrorassembly. The upper surface of the mirror assembly may be coplanar withthe surface of the base. For the case in which the micro-mirror isformed from a silicon wafer, both the base surface and the mirrorsurface may be formed from the original silicon wafer surface (coated bymetal) so that if a polished wafer is used, a high quality mirror iseasily formed. The mirror support structure is suspended above a cavityin the base by micromachined torsional springs. The mirror is separatedfrom the base by etching away the wafer material from between the mirrorsupport structure and the base. The mirror support structure is providedwith a low mass stiffener, and the springs are provided withelectrostatic deflection plates, so that the actuation force is applieddirectly to the springs.

[0020] In an alternative embodiment, magnetic actuation of the mirrorassembly is provided. The mirror assembly includes a magnetic materialthereon to provide a permanent or temporary magnetic moment. A magneticactuator assembly is operative in conjunction with the magnetic materialon the mirror to rotationally drive the mirror. The magnetic materialcan cover all or a portion of a surface of the mirror assembly. Themagnetic material can be applied to a surface of the mirror assembly ina pattern preselected to improve the magnetic and mechanical performanceof the system, such as to minimize moment of inertia and lowering ofresonant frequency. Alternatively, the magnetic material can comprise aconduction coil formed on a surface of the mirror assembly, whereby amagnetic moment is formed when current is established within theconduction coil. The magnetic material can be formed along an edge ofthe mirror assembly, with an electromagnet disposed out of the plane ofthe mirror assembly.

[0021] The advantages of this invention over the prior art lie in thesimplicity of manufacture, the size and performance of the mirrorattainable in this design, and the accessible range of motion. Themirror can be made nearly as large as the starting wafer substrate(however, the sizes contemplated for the preferred embodiment aretypically in the range of 50 μm×50 μm to 3 mm×3 mm). The resonantfrequency depends on the mirror size; for a 600 μm square mirror,resonant frequencies of over 20 kHz have been demonstrated, and withminor design changes, frequencies appropriate for scanning atfrequencies above 30 kHz may be achieved. In one embodiment, the drivemechanism is electrostatic. However, several aspects of the inventionlend themselves well to magnetic actuation. Because the mirror itself issupported over a cavity in the substrate, large angular displacement ofthe mirror and its supporting structure can be achieved whilemaintaining a small gap between the plates of the drive capacitor formedat the supporting springs. The fabrication of the mirror is relativelysimple. The thickness of the mirror is easily controlled and may beadjusted to tune the resonant frequency or change the stiffness of themirror. The surface of the mirror may be metallized for greaterreflectance, or shaped to give it optical power.

[0022] In the process disclosed here, the mirror support structure isformed from the wafer substrate. The excess substrate material (if any)is first removed from the back of the mirror support structure bypatterned etching, thus defining its thickness, mass, stiffness andthermal conductivity, while the mirror surface geometry is defined bypatterned etching from the front. Using the substrate material to formthe mirror support structure has many advantages. The wafers are ingeneral available highly polished and extremely flat, giving goodspecular reflections (for example, Si and GaAs wafers intended forintegrated circuit production are flat and specular). The reflectance ofsuch wafers can be easily made to exceed 90% by metallizing the surface,for example with a thin layer of aluminum. Such a layer can besufficiently thin (less than 0.5 μm) so as not to introduce undesirabletopological features to the mirror surface. This is an advantage of thecurrent invention over mirrors formed by surface micromachining, forexample by electrodeposition of metal or CVD polysilicon, which aregenerally rough and so require a separate polishing step.

[0023] Silicon is a good choice for the substrate because the mechanicalproperties of single crystal silicon are nearly ideal for micro-mirrorapplications. Silicon is light, strong, and stiff, yielding rigidmirrors with low moments of inertia. The process disclosed here, appliedto silicon, can yield a wide range of mirror thicknesses, and evenallows for engineered structures that may be used for the constructionof stiffer yet lighter mirror supports. The fabrication process for thecurrent invention is relatively simple, requiring only a limited numberof steps and mask levels. The springs in the current invention areconducting and serve as the top electrode, eliminating one fabricationlayer. Finally, this invention uses an electrodeposited metal layerwhich makes possible magnetically actuated designs, by choosing amagnetic material (such as nickel or permalloy) for the metal.

[0024] Accordingly, the present invention relocates the driving force,either electric or magnetic, to sites that do not interfere to the samedegree with mirror motion. Also, the present invention provides asuitably large mirror while maintaining a high resonant frequency (lowmass), adequate stiffness, and adequate thermal conductivity. A mirrorof this invention overcomes the problem of obtaining high mirror massand structural rigidity, while also attaining the desired elasticconstants in the springs. The mirror also overcomes the problem ofattaining mirrors with the desired optical properties, including opticalpower.

DESCRIPTION OF THE DRAWINGS

[0025] The invention will be more fully understood from the followingdetailed description taken in conjunction with the accompanying drawingsin which:

[0026]FIG. 1 is a perspective illustration of a torsional micro-mirrorsystem of the present invention;

[0027]FIG. 2A is a plan view of a torsional micro-mirror system of thepresent invention;

[0028]FIG. 2B is a cross-sectional view of the system of FIG. 2A takenalong centerline 7;

[0029]FIG. 3 is a scanning electron micrograph of a micro-mirror systemof the present invention;

[0030]FIG. 4A illustrates a plan view of a further spring embodiment;

[0031]FIG. 4B is a cross-sectional side view of the spring embodiment ofFIG. 4A;

[0032]FIG. 5 is a scanning electron micrograph of a spring embodiment;

[0033]FIG. 6A is a plan view of a further embodiment of a torsionalmicro-mirror system of the present invention;

[0034]FIG. 6B is a side view of the system of FIG. 6A;

[0035]FIG. 7 is a plan view of a further embodiment of a torsionalmicro-mirror of the present invention;

[0036]FIG. 8A is a schematic side view of an embodiment of a torsionalmicro-mirror system incorporating a magnetic actuation assembly;

[0037]FIG. 8B is a schematic isometric view of a further embodiment of amagnetically actuated system;

[0038]FIG. 8C is a schematic isometric view of a further embodiment of amagnetically actuated system;

[0039]FIG. 8D is a schematic plan view of a further embodiment of amagnetically actuated system;

[0040]FIG. 8E is a schematic isometric view of a further embodiment of amagnetically actuated system;

[0041]FIG. 9 is a schematic plan view of a further embodiment of anactuator assembly for a torsional micro-mirror system of the presentinvention;

[0042]FIG. 10A is a schematic plan view of a further actuator assembly;

[0043]FIG. 10B is an image of a torsional micro-mirror such as that inFIG. 10;

[0044]FIG. 11 is a plan view of a further embodiment incorporatingcantilevered springs to actuate torsional motion;

[0045]FIG. 12 is a schematic plan view of a multi-axis torsionalmicro-mirror according to the present invention;

[0046]FIG. 13A is a schematic cross-sectional view of a furtherembodiment of a multi-axis torsional micro-mirror having wire-bond wirejumpers according to the present invention;

[0047]FIG. 13B is a schematic plan view of a multi-axis micro-mirror ofFIG. 13A having integrally fabricated contact structures;

[0048]FIG. 13C is a schematic view of a multi-layered torsional springcontaining multiple electrical paths to the mirror;

[0049]FIG. 14A is a schematic cross-sectional view of a torsionalmicro-mirror incorporating a damping material surrounding the springs;

[0050]FIG. 14B is a schematic isometric view of a torsional micro-mirrorincorporating damping material at several positions along the movingedge of the mirror;

[0051]FIG. 14C is a schematic cross-sectional view of a torsionalmicro-mirror with a damping coating on the springs;

[0052]FIG. 14D is a schematic cross-sectional view of a torsionalmicro-mirror with high damping layers within the springs;

[0053]FIG. 15A is an image of a biaxial micro-mirror with wire-bond wirejumpers and damping with vacuum grease;

[0054]FIG. 15B is an image of the biaxial micro-mirror of FIG. 15Aillustrating a magnetic foil laminated to the back to provide a magneticmoment;

[0055]FIG. 16 is a schematic isometric view of a cantileveredmicro-mirror according to the present invention;

[0056]FIG. 17 is schematic cross-sectional view of a micro-mirrorincorporating optical sensing;

[0057]FIG. 18A is a schematic plan view of a torsional spring FIG. 18Bis a schematic plan view of a further embodiment of a torsional spring;

[0058]FIG. 18C is a schematic plan view of a further embodiment of atorsional spring;

[0059]FIG. 19 is a schematic plan view of a micro-mirror with taperedsupports;

[0060]FIG. 20 is a schematic plan view of a micro-mirror with springshaving necked down regions;

[0061] FIGS. 21A-21G are schematic cross-sectional views illustrating afabrication process for a micro-mirror according to the presentinvention;

[0062]FIG. 22A is a schematic side view illustrating the step in thefabrication process of providing a mirror;

[0063]FIG. 22B is a schematic side view illustrating the step in thefabrication process of providing a mirror with an adhesive or otherattachment layer, support layer, and a reflecting layer;

[0064]FIG. 22C is a schematic side view illustrating the step in thefabrication process of providing a mirror with an adhesive or otherattachment layer and a support layer and a curved reflecting layer;

[0065]FIG. 22D is a schematic side view illustrating the step in thefabrication process of providing a stress compensating layer; and

[0066] FIGS. 23A-G are schematic side views illustrating the fabricationprocess of providing a backside patterning.

DETAILED DESCRIPTION OF THE INVENTION

[0067]FIG. 1 shows a perspective illustration of a torsionalmicro-mirror system. Support posts 1 are mounted to a base 2. A mirror 3and mirror support structure 4 are provided between the posts and aresuspended by torsional springs 5. A cavity 6 formed in the base 2 belowand around the mirror support structure 4 is provided to facilitate therotation of the mirror. The deeper the cavity 6, the greater therotation angle of the mirror. In some embodiments of the invention, thecavity extends entirely through the base so that the base does not limitthe rotation at all. It should also be noted that although the mirrorshown in FIG. 1 is rectangular, the mirror support structure and mirrorsurface may be any practical shape including round, ovoid, or octagonal,and may be selected to reduce the mass of the mirror support structure,while still yielding a satisfactory mirror area.

[0068]FIG. 2A shows a plan view of one preferred embodiment, and FIG. 2Bshows the cross section of the system shown in FIG. 2A, taken alongcenterline 7. The invention consists of a mirror surface 3, parallel inthis case with the surface of base 2 and suspended above an opening 6 inthe base by two torsional springs 5. The springs 5 are collinear andaligned with the mirror support structure 4 centerline 7, and are offsetfrom the substrate and mirror surface by posts 1. The mirror surface 3may be formed by vapor deposition of a metal such as Al onto mirrorsupport structure 4. Metallized regions 8 on the base surface, under thespring 5 and offset laterally from the centerline 7 form one plate ofthe electrostatic drive capacitor, and the spring 5 serves as the otherplate. Electrical contact to the drive capacitor plates is made bymicrofabricated conduction lines, and isolation between the conductorsand the base is achieved by an oxide layer 13 formed on the surface ofthe substrate. The posts 1 offsetting the springs 5 from the substratesurface set the size of the capacitor gap.

[0069] A preferred embodiment of the invention is designed to beoperated at the resonance frequency of the device, which depends on thegeometry and thickness of the mirror and the shape and material of thesupports, as will be discussed later. In brief, necked down regions 9reduce the spring constant of the springs 5 and reduce the driving forcerequired for actuation. The reduced spring constant also reduces theresonant frequency. The extent of motion in response to a given drivevoltage is determined by the stiffness of the springs 5, includingnecked down regions 9, and the size of the gap 10 between springs 5 andplates 8, the area of the plates 8, and the quality factor Q of theresonant structure. The maximum displacement is limited by the angle thetorsional spring can twist through before the edge of the springcontacts the base. A plan view scanning electron micrograph of a mirrorsystem formed in this way is shown in FIG. 3.

[0070] There are several key advantages of this invention over the priorart. First, this invention uses electrostatic actuation, applied to thesprings 5. FIG. 4A and B illustrate one spring design, in which it canbe seen that electrostatic force pads 11, 12, placed on an oxide 13 uponbase 2 (not behind the mirror), are connected to pads 14 and 15. FIG. 4Bshows a cross sectional view along line 16, and FIG. 5 shows a scanningelectron micrograph of a spring system formed in this way. Spring 5,(metallic in this embodiment), is connected to pad 17. An electrostaticdriving force can be applied between the spring 5 and the fixed forceplate 12 by biasing pad 15 (in electrical contact with 12 through wiretrace 18) with respect to pad 17 (in contact with spring 5 through trace19), and similarly, a force can be applied between fixed force plate 11and spring 5 by biasing pad 14 with respect to pad 17. Alternatelybiasing the pads with a periodic potential at or near the resonantfrequency of the system will excite resonant motion of the mirror.Alternatively, the resonant motion may be excited by biasing only oneplate and in such a case the other plate may be used for sensing themotion of the mirror by measuring the capacitance changes as the gapchanges. For example, an AC bias may be applied between spring 5 andplate 11 at or near the resonant frequency to induce motion, and a DCbias voltage applied between plate 12 and spring 5. The changes incapacitance between spring 5 and plate 12 owing to the motion of thespring 5 may be sensed by monitoring the current at pad 12. On a givenmirror support structure, one or both of the springs 5 may be biased andor sensed.

[0071] Note that in the embodiments shown herein the force pads are notbehind the mirror, thus the mirror motion is not limited by the problemof the mirror contacting the force pad. The spring itself may contactthe base, which will ultimately limit the range of motion of the mirror;however, the spring and force pads may be designed to yield a muchgreater range of motion, as we will show in the various preferredembodiments. Additionally, the fixed force plates 11, 12 may be designedso as to prevent electrical contact between the spring 5 and the fixedplates 11, 12, for example by coating the plates with a dielectric or bylimiting the width of the plates so that the distance from thecenterline to the plate edge is less than the product of one half thespring width and the cosine of the maximum deflection angle.

[0072] For certain applications, it may be desirable to modify thedesign by bonding external mirrors to the mirror support structure in aseparate step. A mirror may be physically bonded to mirror supportstructure 4 (FIG. 2). In a different design (FIG. 6) the spring 5 alsoserves as the mirror support structure, and extends across the cavity 6.The mirror is bonded directly to this spring. These external mirrors 20could be made of materials that are not convenient to include in thefabrication process, or could be made of mirrors having optical power.The approach shown in FIG. 6 has the additional and desirable featurethat the cavity in the base wafer may be formed by etching only from thefront side of the wafer.

[0073] Another embodiment of the invention uses a differentelectrostatic capacitor design for the drive, and is shown in FIG. 7. Apair of electrodes 21 is formed along the edges of the mirror parallelto the rotation axis. A second pair of electrodes 22 is formed on thebase along the edges of the cavity nearest the mirror electrode. Theforce resulting from an applied voltage is proportional to thecapacitance between the electrodes, which is inversely proportional tothe gap between them. In this configuration no gap is required betweenthe torsional springs 5 and the base 2, so the fabrication of the postsmay be eliminated if desired. The length of the freely rotating portionof the springs 5 may be controlled by enlarging the width of the cavity6 so as to increase the distance 23 between the mirror support structure4 and the base 2. The distance 23 may differ from the distance 24between electrodes 21 and 22.

[0074] A variation of the design involves magnetic actuation of thestructure (FIGS. 8A though 8D), again without restricting the freedom ofmotion of the mirror. In this case, magnetic material 25 is applied tothe mirror support structure (FIG. 8A) partially or totally covering thestructure surface in order to give it a magnetic moment. The drivingfield is provided by an external electromagnet 26 which exerts a torqueon the magnetic structure, causing the mirror support to rotate. Theelectromagnet may be placed at a sufficient distance from the mirror toallow free rotation. However, if only limited motion is required, themagnetic coil may be placed closer to the mirror, thus reducing the coilcurrent necessary to achieve actuation.

[0075] The magnetic material may be patterned to improve the magneticand mechanical performance of the device. Specifically (FIG. 8B) thematerial may be confined near the axis of rotation so as to minimize itsmoment of inertia and avoid lowering the resonant frequency, and may beshaped into one or several elongated structures 27 so that the preferredmagnetic axis is perpendicular to the rotation axis.

[0076] The magnetic material may either have a permanent magnetic momentor temporarily acquire a moment upon the establishment of an externalmagnetic field, in which case the torque is due to the shape anisotropyof the magnetic structure. The electrodeposited layer comprising thesprings may be chosen to be of a magnetic material, for example (but notnecessarily) permalloy, in which case the magnetic structure may beformed in the same operation as the fabrication of the springs, orincorporated into the design of the springs themselves. Alternatively,two different materials may be used for the springs and magneticstructures, which may be formed in separate steps. Alternately, themagnetic material may be separately deposited or laminated onto thestructure, in which case the magnetic material need not be formed by oreven be compatible with standard fabrication processes, allowing the useof, for example, ceramic ferromagnets.

[0077] Another approach comprises patterning of a conduction coil 28(FIG. 8C) onto the mirror support structure, which creates a magneticmoment when current is established. If multiple turns are used, abridging structure 29 may be fabricated to connect the center of thecoil to the electric leads. The magnetic moment established when currentflows in the coil interacts with the field of a small permanent magnet30 to rotate the mirror support structure to the desired angle.

[0078] Yet another approach to magnetic actuation uses a reluctancecircuit approach (FIG. 8D). One or more external electromagnets 31 aremounted either slightly displaced above the mirror structure or at anangle to it. Upon the establishment of a current in the coil, themagnetic material 32 is pulled towards the center of the coil, formingpart of the electromagnet core. In this case, additional magneticmaterial 33 may be deposited on the base 2 for the attachment of theexternal coils 31 and used to define the shape of the applied magneticfield. The electromagnet may be fabricated on the wafer using standardtechniques.

[0079] A fifth approach to magnetic actuation uses a permanent magnetattached to the mirror structure to provide a magnetic moment normal tothe mirror surface. See FIG. 8E. In this configuration a magnetic fieldapplied parallel to the mirror surface and orthogonal to the axis ofrotation exerts a torque on the permanent magnet causing the mirror torotate. The field could be produced, for example, by a pair ofelectromagnet coils placed on either side of the mirror along an axisorthogonal to the rotation axis, with the coil axis parallel to thesurface of the mirror.

[0080] Yet another embodiment of the invention suspends the mirror aboutan axis of rotation 34 displaced from its centerline 7, as shown in FIG.9, allowing greater linear displacement on one side of the axis ofrotation than the other. In this embodiment a large movable capacitorplate 35 is arranged to push on the springs 5. The large fixed plate 36is biased with respect to plate 35, thereby generating a large forcewhich is transferred to the springs. The force of plate 35 may also betransferred directly to the mirror by an arm 37 as shown in FIG. 10A,which may or may not be attached to the mirror. The drive of themechanical actuator may be electrostatic, magnetic, or piezoelectric. Animage of a scanning electron micrograph of such a device is shown inFIG. 10B.

[0081] Alternatively, one or both springs may be split into two elements99 along the axis of rotation 7 (FIG. 11) in order to provide additionalelectrical paths onto the mirror support assembly. In this embodiment,rotational compliance of the springs may be replaced by bending ortwisting of the support posts 100 at the mirror support platform. Thishas the further advantage that the rotational motion of the mirrorsupport platform can result from cantilevered bending of the springs. Atthe fixed base end, the springs may be made as wide as desired toincrease the electrostatic force. Applying a voltage between the springsand the capacitor plates on the fixed base, alternately on each side ofthe centerline 7, causes the springs to bend and push down on thesupport platform. By attaching the springs to the platform near the axisof rotation, large angular displacement can be achieved for very smallvertical motion. This embodiment improves on a device such as describedby Dhuler in several important aspects. First no bearing or additionalelement is required for supporting the moving platform, simplifying thefabrication and eliminating wear associated with surfaces moving againsteach other. Second, the springs and supports described in this inventionmay serve as electrical paths onto the mirror support platform.

[0082] Video images require sweeping in two orthogonal directions, butthe second sweep direction need not move faster than the frame rate,ranging from 30-180 Hz. To obtain images, two separate mirrors could beused, rotating about orthogonal axes, or a single reflecting surfacecould be made to scan both directions. A single mirror that scans twoorthogonal directions is achieved either by mounting the currentinvention on a scanning platform, or modifying the design so thereflecting surface is supported within a gimballed frame, and made toscan in both directions. The actuating mechanism for the two directionscould be direct or indirect electric or magnetic force, or anycombination thereof.

[0083] A multi-axial micro-mirror may be formed using the designs andprocesses described herein. The actuation mechanism for the twodirections is direct or indirect electric or magnetic force, or anycombination thereof. FIG. 12 illustrates one possible multi-axialdesign: a first pair of springs 38 is used for rotation of the mirrorsupport structure 4 along one axis, and a second pair of springs 39 isused for rotation along a second axis which in this case isperpendicular to the first axis. The first pair 38 joins the base 40 toa movable support frame 41; this support frame 41 is connected to themirror support structure 4 by the second pair 39. (Other pairs andadditional movable supports may be added that can be designed to operateat resonance or in a bi-stable mode with the advantage of providingaiming or alignment of the micro-mirror system. Other pairs may also beuseful in distortion correction.) The deflection voltage is supplied tothe pair 38 through bias applied to pads 42 and 43, and through biasapplied through pads 44 and 45. The deflection voltage for pair 39 issupplied through pair 38 by traces 46 and 47. Thus, by relativelybiasing the two springs in pair 38, bias to the pair 39 may be attained,without addition of further conduction paths to the moving parts.

[0084] If separate electrical contacts are to be provided for the innermirror of a gimballed structure, they may be combined with themechanical support of the outer mirror support frame, or may be runthrough separate structures bridging the gap between the inner and outersupport structures, as shown in FIG. 13. These separate structures mayor may not contribute to the mechanical properties of the system. Thestructures may either be added as part of the packaging process, such asthe addition of wire-bond wire jumpers 48 in FIG. 13A, or may befabricated along with the rest of the structure. An example ofintegrally fabricated contact structures which minimally impact themechanical properties is shown in FIG. 13B. Soft serpentine springs 49bridge the gap, and connect through traces 50 to the inner springs 39.The outer springs 38 are separately connected to the drive pads 11 and12 by traces 52 and 53 respectively. Multilayered spring supports mayalso be used to increase the number of separate electrical paths to theinner mirror. FIG. 13C shows such a spring; an insulating layer 54separates the upper conducting layer 55 from the lower conducting layer56. Each conducting layer connects to a separate pair of traces (notshown).

[0085] It may be desirable for the inner and outer support structures tohave different damping characteristics. For example, in a scanningdisplay, the line scanning structure may be resonantly driven andbenefit by low damping (high Q), while the frame scanning structure isdriven linearly at low frequency and benefits from high damping foruniform motion. Thus, the device can be operated in a vacuum package tominimize the air damping of the fast mirror, with specific damping meansprovided for the slow scanning structure. For example, a dampingmaterial 57 may be applied to the slow moving structure or the springs(FIG. 14A). This damping material may consist of a liquid, gel, or softsemi-solid surrounding the springs, with or without an enclosure toconfine it. Many materials are suitable for this purpose, includingvacuum grease such as Dow Corning DC 976 or Apiezon N type, RTVsilicone, or spin-on polymers used in the fabrication process such aspolyimide or photoresist (for example Shipley AZ1308). One embodiment isthe application of a drop of DC 976 57 to the substrate so that itencloses the springs (FIG. 14A). Optionally, the damping material 98 mayinstead be applied along the moving edge away from the spring (FIG.14B), so that it bridges the gap between the moving support element andthe fixed base of the device. The damping material may be appliedanywhere along the gap between the moving component and the fixedcomponent, or between two components moving at different velocities, forexample, the mirror platform and the gimballed outer frame of a biaxialembodiment. An alternative is to coat the springs 38 with a high dampingcoating such as photoresist 58 (FIG. 14C). Another alternative is toenclose a high damping material within the springs, for example bymaking a multilayer structure incorporating high strength layers 59surrounding high damping layers 60. Damping may also be provided bymechanically attaching damping devices (dashpots) between the movingstructure and the base.

[0086]FIG. 15A is a micrograph image of a biaxial scanning displaydevice. The inner mirror support platform 4 is the line scanner, drivenby an electrostatic resonant drive. The oxide has been removed from itssurface and it has been coated with aluminum to form the mirrorreflective surface. The electrical ground lead contact is made throughthe outer frame supports 38 and the drive voltage contacts are made bywire-bond wire jumpers 48. The outer frame is the frame scanningdirection and is driven magnetically to give a slow linear sweep and afast retrace. Harmonic oscillations in the motion are damped by theapplication of approximately 0.005 microliters of DC 976 vacuum grease57 to dampen the slow axis springs 38. The magnetic moment is providedby magnetic foil 25 laminated to the back of the structure, coveringhalf the area and placed orthogonal to the axis of rotation (FIG. 15B).This device is then mounted on a small electromagnet and packaged invacuum.

[0087] In a preferred embodiment of a device as shown in FIG. 15, themirror support platform is approximately 1 mm×1 mm and has a resonantfrequency ranging from 7 to 15 kHz. The resonant frequency of the outerframe is 150 to 700 Hz. The outer frame is driven at 60 Hz. The deviceis packaged in vacuum above an electromagnetic coil, preferably in a T05package containing optics.

[0088] In another embodiment of the invention, shown in FIG. 16, themirror support structure 92 is connected to the base 40 along one edge94 by one or several metal cantilever springs 93, allowing the mirrorsupport structure to rotate out of the plane of the wafer. The stiffnessof the support springs 93 depends on the total aggregate width, but thiswidth may be distributed among as many supports as desired (five in FIG.16), each providing a separate electrical path onto the mirror supportstructure. Other devices, for example a torsional MEMS mirror such asdescribed herein, a CMOS circuit, for example the drive circuit, orboth, may be fabricated on or grafted onto the platform 92. The numberof possible electrical contacts is limited only by the length of theedge 94 and the minimum practical width and separation of the springs93. This embodiment is an improvement to the mirror disclosed in theliterature by Miller, in that the support springs are conducting and mayprovide multiple distinct electrical contacts to elements on the mirrorsupport platform. In addition, the platform in the present invention maybe formed of the original wafer, which may be a single crystalsemiconductor, and may have a desired thickness up to the full thicknessof the original wafer. Thus, it may comprise any desired CMOS circuit ordevice, which may be fabricated on the substrate prior to the release ofthe cantilevered platform.

[0089] The motion of the platform 92 may be used either to set the angle95 between the platform 92 (and any devices carried upon it) and thebase 40 (and any other devices attached to it), for example for thepurpose of optical alignment, or to sweep the angle, for example for adisplay. The cantilever may be moved into place mechanically, forexample during the fabrication or packaging of the device, and lockedinto place, to fix the angle. Alternatively, magnetic material may beapplied to the front, back, or both surfaces of the platform and amagnetic field may be applied to it during operation to rotate it to thedesired angle. In this case, a DC magnetic field bias may be applied toset the center angle of motion, and an AC field superimposed on it tosweep the angle. Either the AC or DC component may be zero. In anothervariation, mechanical elements such as levers may be provided to set theangle during operation.

[0090] Sensors may be added to the mirror to detect its position and theextent of the motion and provide feedback for the drive electronics. Onesensor design consists of capacitors similar to but separate from thedrive pads, with detection of the current changing as a function ofmirror position as previously described. If magnetic material is presenton the moving structure, a magnetic sensor, for example a pickup coil,may be placed in close proximity to detect the mirror position. Opticalsensing may also be utilized, as shown in FIG. 17. A light source 61provides a focused beam 62 which is reflected off the mirror 3 anddetected by a detector 63. As the mirror rotates, the intensity of lightincident on the detector 63 changes and can be correlated with mirrorangle. The angle of incidence 64 can be chosen so the sensing light doesnot interfere with the display illumination, or an infrared source maybe used. Alternatively, the detection may be made from the back of themirror structure by reflecting the light off the back of the mirrorsupport structure 4. Optionally, the mirror position may be determinedby sensing the light incident on it for display illumination. Detectionmay also be made by allowing invisible radiation such as infrared lightto pass through a specifically designed mirror surface coating to adetector mounted on the back of or underneath the device.

[0091] Support and Spring Design

[0092] For electrostatic actuation, the force is proportional to thesquare of the applied voltage and inversely proportional to the squareof the gap 12 (FIG. 2) between the drive capacitor plates. Making thegap smaller or the spring wider reduces the maximum angle of rotationbut increases the applied torque, while making the gap 12 larger or thesupport narrower allows greater motion but reduces the torque resultingfrom a given applied voltage. At the maximum displacement, at least someportion of the spring 5 touches the base 2. The present invention isdesigned so that the mirror itself is free to rotate to any degreewithin the elastic limits of the springs, and the displacement islimited by the rotation of the springs in relation to the base. For afixed gap height g, if at any given point z along the length of thespring its width is w(z), then the maximum rotation allowed at thatpoint is approximately

sin(φ)=2 g/w(z).

[0093] The angle of rotation varies along the length of the spring, fromapproximately zero at the connection to the support post to the maximumat the mirror support structure, and the spring shape can be designed totake advantage of this, for example by making the spring wider near thefixed end and narrower at the mirror support structure, allowing agreater angle of rotation while still allowing for a large plate areaand thus allowing for application of a larger torque than would beobtained in a spring of uniform cross section.

[0094] Several possible spring designs are shown in FIG. 18. Thestiffness of each design can be calculated from standard mechanicalexpressions, for example as found in Mark's Handbook of MechanicalEngineering. T. Baumeister, editor in chief, Mark's Standard Handbookfor Mechanical Engineers, 8th ed., McGraw-Hill Book Company, New York,1978, Section 5. For the uniform cross section spring shown in FIG. 18Athe torsional stiffness Kt is given by$K_{t} = {\frac{G}{3.51}\left( \frac{b^{3}h^{3}}{b^{2} + h^{2}} \right)1}$

[0095] where G is the shear modulus, l, b, and h are the length, width,and depth of the member respectively, and the numerical constant dependson the aspect ratio of the cross section. For non-uniform cross sectionsprings, the reciprocals of the stiffness of individual elements add togive the reciprocal total stiffness:$K_{total}^{- 1} = {\sum\limits_{i}{K_{i}^{{- 1}\quad} \cdot 2}}$

[0096] For example, introducing a necked down region, as in FIG. 18B,reduces the stiffness to:${K_{total} = \frac{K_{1}K_{2}}{K_{1} + K_{2}}},3$ where${K_{t} = {\frac{G}{3.5\quad l_{i}}\left( \frac{b_{i}^{3}h^{3}}{b_{i}^{2} + h^{2}} \right)}},4$

[0097] since the wide part 65 of the spring 5 and the necked down part66 of the spring 5 have different lengths and widths, but the thicknessof the electrodeposited layer and the material properties are the samefor both. This type of design makes the device easier to actuate, butalso reduces its resonance frequency. For a tapered support as shown inFIG. 18C, the stiffness is given by:${K_{t} = {\frac{{Gh}^{3}}{3.5l}\frac{\left( {b_{\max} - b_{\min}} \right)}{\ln \left( {b_{\max}/b_{\min}} \right)}}},5$

[0098] where the width varies from b_(max) at the substrate support postto b_(min) near the mirror.

[0099] For the case of a mirror support structure in which the rotationaxis coincides with the center of mass, the resonant frequency dependson the moment of inertia of the mirror element, J_(t),${J_{t} = \frac{\rho \quad L^{3}{Wt}}{12}},6$

[0100] where ρ is the density of the mirror support structure, L is itslength, W is its width (parallel to the axis of rotation) and t itsthickness. The resonance frequency is then:$f = {\frac{1}{2\pi}{\sqrt{\frac{K_{t}}{J_{t}}} \cdot 7}}$

[0101] For a given limiting angle and resonant frequency, the tapereddesign (FIG. 18C) allows the gap g to be reduced by a factor of:${\frac{g_{tapered}}{g_{uniform}} = \frac{\ln \quad \xi}{\xi - 1}},8$where $\xi = {\frac{b_{\max}}{b_{\min}}9}$

[0102] over the gap for the uniform cross section design (FIG. 18A). Foraspect ratios ξ less than or approximately equal to 3, the limitingangle is imposed by contact between the spring and base at the narrowend of the spring. FIG. 19 illustrates a micro-mirror with taperedsupports.

[0103] Another design is shown in FIG. 20, in which the spring 5 has twonecked down regions 67, 68, which facilitate a high rotation angle. Thenecked down regions are separated by a wider region 69 which comprisesthe moving force plate 70 section of spring 5. The force plate 70 ofspring 5 is attracted alternately to plates 11 and 12 and can rotatethrough an angle φ given by the distance 67 (denoted w₆₇) and the gap(g), by the equation: sin(φ)=2 g/w₆₇. The mirror support structure 4 cancontinue to rotate through an additional angle which is limited by theelastic limits of section 68 of spring 5. In general, for rotations ofsection 67 less than φ and equal to α, the total rotation γ is given by:

γ=αw ₆₈ /w ₆₇.

[0104] By selecting the relative lengths of the necked down regions,large angular displacements γ are possible with only a small movement αin the force plate 70 integral to spring 5.

[0105] Fabrication

[0106]FIG. 21 shows the fabrication process. A polished wafer 71,preferably Si, is first coated on both sides with a material 72 on thefront and 73 on the back that is resistant to etches of the wafermaterial. For silicon, this material may be silicon nitride, silicondioxide, or other films known in the art. For the case of silicondioxide to be formed on Si, the wafer may for example be oxidized toform a surface layer of silicon dioxide 72, 73 on both sides of thewafer, or the wafer may be coated by chemical vapor deposition, or byother means. After application of coating 72, 73, the wafer 71 andcoatings 72,73 are then patterned on both sides with registeredalignment marks and etched to define the marks in the crystal. Thesemarks, formed on both sides of the wafer, permit registration offeatures on the front and back (registration marks are not shown in FIG.21).

[0107] Metal films, for example of chromium, gold, and titanium/tungstenalloy, are deposited on the front coated surface 72, and are patternedand etched to form pads 74 that provide the electrical contacts andanchors for the mechanical structures. The coating 73 is patterned andetched to act as a mask for wafer etching. The back of the wafer is thenetched to form a membrane with surface 75 having thickness in the rangeof 20 μm to 200 μm. A typical thickness is 60 μm. The coating 72 on thefront surface is then patterned and etched to form groove openings 76 inthe coating which will serve later in the process as an etch mask forthe separation of the mirror support structures 4 from the base 2. Theinitial coatings may also include or serve as the final mirror surface.

[0108] A release layer 77 of photoresist or other material is applied tothe front surface and patterned with holes 78 to expose the metalanchors 74. After heat treatment, thin (0.05 μm to 0.5 μm) layers of ametal or sequence of metals such as chromium, gold and titanium/tungstenalloy 79 are deposited on the front surface. Photoresist is then appliedand patterned to form a mask 80 for the electrodeposited structures. Ametal layer 81, which may be nickel, is deposited by electroplating onto the exposed regions 82 of metal layers 74 and 79. The thickness ofmetal layer 81 is in the range of 0.5 μm to 10 μm; layer 81 constitutesthe spring 5 in the plan views described earlier. The mask 80 andrelease layer 77 are removed by dissolving the layers in solvents orpreferential etches. This process also removes sections of intermediatemetal layers 83 (of metal layer 79) that are not reinforced by theelectroplating.

[0109] The wafer is diced, and the mirror support structure 4 isseparated from the surrounding base 2 by etching both from the front,through the grooves 76 defined in the etch masks 72 and 73, and from theback by etching surface 75, resulting in the formation of cavity 6surrounding the mirror support structure 4. The mirror support structure4 is thus joined to the base solely by the metal torsional springs 5.The final thickness of the mirror support structure 4 depends on theduration of the two etch steps and can be selected to yield structureswith thickness in the range of 10 to 200 μm or more (as large as thewafer thickness if needed). Typical final support structure thickness is30 μm.

[0110] The final step comprises addition of a mirror, either byproviding a metal or dielectric coating 84 (FIG. 22A) of the mirrorsupport structure through a mask (for example with evaporated orsputtered aluminum with thickness in the range of 500 to 2500 angstrom),or by bonding a finished mirror to the support structure. FIG. 22B showsa mirror bonded to the mirror support structure, comprising an adhesiveor other attachment layer 85, a glass or other substance 86 as is usedin the art of mirror formation to support a reflecting layer, and a flatmirror (reflecting) layer 87. This invention additionally comprises thecapability to use a mirror comprising a curved surface formed inmaterial 86 and coated with a reflecting layer 88, as shown in FIG. 22C.This surface on material 86 may be either convex or concave, oraspherical. Additionally, the layers 87 or 88 may comprise a binaryoptical element of a diffractive or refractive nature, or a holographicelement.

[0111] Any of the coatings used in the fabrication process (for examplethe silicon oxide layer or the optical coatings) may have a high degreeof internal stress. If the mirror support structure is sufficientlythin, this stress could induce curvature in the structure. Thiscurvature may be useful, for example for shaping the optical surface, ormay be undesirable. In the latter case, if it is not possible ordesirable to remove the stressed film, additional layers 89 of equalstress may be deposited on the back face of the structure to compensatefor the stress and restore the surface flatness (FIG. 22D). Alternately,the compensating layer may be applied to the front of the supportstructure, under the mirror, in which case the compensating layer stresswould be equal and opposite to the stress already present. If thereflectance of the compensation material is sufficient (e.g. if chromiumis used), it may serve as the reflective surface as well.

[0112] The mirror thickness is determined by the duration of the etchused to remove the excess material from the back. The choice ofthickness results from a tradeoff between mechanical stiffness, momentof inertia, and thermal conductivity. Thicker mirrors are stiffer, andso deform less in use, but the additional mass results in a highermoment of inertia which lowers the resonant frequency. By addingbackside patterning of the mirror support structure to the fabricationprocess as shown in FIG. 23, an engineered mirror support structure canbe made, so as to yield a support which is stiff yet low in mass. Forexample, by adding a patterned mask 90 to the back etching, support ribscould be fabricated 91 on the back of the mirror, perpendicular to therotation axis, adding stiffness but minimal weight. Alternately acorrugated, honeycomb, or other structure may be formed by etching apattern of wells into the back of the mirror support structure. Anothermethod that can be applied when the substrate is a single crystal (suchas Si) is to pattern the back surface with an array of square openingsaligned to the crystal axis, followed by etching in a selective etch(such as potassium hydroxide etching of Si) which etches {111} planesmuch more slowly than other planes of the crystal. The etching processself terminates when the {111} planes are fully exposed, resulting in apyramidal etch pit of highly controlled depth. In this way a corrugatedstructure with a precise resultant mass is formed in the mirror supportstructure, without the need for a buried etch stop layer.

[0113] Structures having a magnetic moment, for use in magneticactuation, may be electroplated in the same manner as the springs. Ifthe material used for the springs is magnetic, they may be deposited atthe same time as the springs. If two different materials are to be used,the magnetic structures may be electroplated in a separate step using asimilar processing sequence to that described above. A third option forapplying magnetic material to the structure consists of attaching apreformed magnetic element to the completed device by means of anadhesive such as cyanoacrylate adhesive or Crystal Bond wax.

[0114] The invention is not to be limited by what has been particularlyshown and described, except as indicated by the appended claims.

We claim:
 1. A torsional micro-mechanical mirror system comprising: abase having a cavity formed therein; a mirror assembly including amagnetic material thereon to provide a magnetic moment; a torsionalmirror support assembly comprising at least one torsional springsupporting the mirror for rotational movement over the cavity; and amagnetic actuator assembly operative in conjunction with the magneticmaterial on the mirror to rotationally drive the mirror.
 2. The systemof claim 1 , wherein the magnetic actuator assembly comprises anelectromagnet disposed to exert a driving force on the magneticmaterial.
 3. The system of claim 1 , wherein the magnetic materialcovers a surface of the mirror facing the cavity.
 4. The system of claim1 , wherein the magnetic material is applied to a surface of the mirrorassembly in a pattern preselected to improve the magnetic and mechanicalperformance of the system.
 5. The system of claim 1 , wherein themagnetic material is applied to a surface of the mirror assembly near anaxis of rotation of the mirror assembly in a pattern preselected tominimize moment of inertia and lowering of resonant frequency.
 6. Thesystem of claim 1 , wherein the magnetic material comprises a conductioncoil formed on a surface of the mirror assembly, whereby a magneticmoment is formed when current is established within the conduction coil,and the actuator assembly includes a permanent magnet having a magneticfield disposed to interact with the magnetic moment of the conductioncoil.
 7. The system of claim 1 , wherein the magnetic material is formedalong an edge of the mirror assembly, and the actuator assemblycomprises an electromagnet disposed out of the plane of the mirrorassembly.
 8. The system of claim 1 , wherein the magnetic material has apermanent magnetic moment.
 9. The system of claim 1 , wherein themagnetic material has a temporary magnetic moment upon application of amagnetic field.